<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>时钟案例</title>
    <style>    html, body {
        margin: 0;
        padding: 0;
        height: 100%;
        background-image: linear-gradient(to bottom right, red, gold);
    }

    .box {
        width: 400px;
        height: 250px;
        background-color: rgba(255, 255, 255, 0.6);
        border-radius: 6px;
        position: absolute;
        left: 50%;
        top: 40%;
        transform: translate(-50%, -50%);
        box-shadow: 1px 1px 10px #fff;
        text-shadow: 0px 1px 30px white;
        display: flex;
        justify-content: space-around;
        align-items: center;
        font-size: 70px;
        user-select: none;
        padding: 0 20px;
        -webkit-box-reflect: below 0px -webkit-gradient(linear, left top, left bottom, from(transparent), color-stop(0%, transparent), to(rgba(250, 250, 250, .2)));
    }  </style>
</head>
<body>
<div class="box">
    <div id="HH">00</div>
    <div>:</div>
    <div id="mm">00</div>
    <div>:</div>
    <div id="ss">00</div>
</div>
<script>    window.addEventListener('load', function () {
    clock();
    setInterval(clock, 1000);
});

function clock() {
    let dt = new Date();
    let HH = dt.getHours();
    let mm = dt.getMinutes();
    let ss = dt.getSeconds();
    console.log(dt, HH, mm, ss);
    document.querySelector('#HH').innerHTML = padZero(HH);
    document.querySelector('#mm').innerHTML = padZero(mm);
    document.querySelector('#ss').innerHTML = padZero(ss);
};

function padZero(n) {
    console.log(n);
    return n > 9 ? n : '0' + n;
}  </script>
</body>
</html>